Research Director of the Adam Smith Institute, Sam Bowman, participated in a panel of expert economists who were asked to rate an independent Scotland's currency options for the Financial Times:

Sam Bowman, research director at the Adam Smith Institute Currency union with the UK: 5

In many ways a currency union would be business as usual for Scotland and the least problematic short-term option. However, EU regulations may still require RBS and Lloyds to domicile in the City even under a currency union. In addition, Scottish sovereign debt may trade at artificially low rates thanks to the implicit backing of the BoE, encouraging unsustainable borrowing.
Most seriously, without fiscal transfers between Scotland and the rest of the UK, there is a danger that Scotland would not be able to do anything in response to demand-side shocks to the economy, potentially resulting in prolonged and harmful periods of deflation for Scotland.
Sterlingisation: 9
Sterlingisation could be Scotland’s best option in the medium to long term, especially if combined with banking reforms allowing banks to issue their own promissory notes, backed by the pound sterling on a fractional basis. With no central bank to act as a lender of last resort, banks would be required to make private provision for such facilities. International evidence from the dollarised Latin American states, notably Panama, Ecuador and El Salvador, suggests that this would improve bank soundness by eliminating moral hazard.
Without restrictions on note issuance (and the monopoly protections that encourage excessive issuance), banks would expand and contract their balance sheets in a countercyclical fashion, offsetting changes in velocity with immediate changes in the money supply, reducing the risk of the sort of demand-side recession that took place globally in 2008. This radical option may prove difficult to transition to in the short term, however.
A separate currency: 7
The option of an independent Scottish currency has been unfairly maligned. A free-floating currency would indeed be at risk of speculative attack but with the right mandate it could have substantial benefits as well. For example, were the Scottish central bank to target nominal gross domestic product instead of following an inflation target, a Scottish currency would provide a stable macroeconomic environment that adjusted to shocks automatically, keeping nominal spending levels (or aggregate demand) constant (in a similar way to the free banking option, albeit through a different mechanism). By keeping spending constant along a predictable growth level, an independent Scottish currency would lose purchasing power in recessions but would avoid the “musical chairs” problem of sharp drops in nominal GDP leading to unnecessary structural unemployment.
A free-floating currency would be at risk of the Dutch disease, however, with Scotland’s substantial resource wealth making its other export sectors relatively uncompetitive.

Joining the euro: 2
Joining the euro carries the same risks as a currency union with the UK but on a greater scale. The eurozone is already far from being an optimal currency area, and it is easy to imagine shocks to the Scottish economy (such as a drop in oil prices) that would be barely felt in the rest of the eurozone and hence would receive no policy response.
On top of these potential dangers the ECB has already proven itself to be a badly run institution in practice, strangling the eurozone, stifling recovery and pushing up unemployment with tight money. There is almost nothing positive to be said for this option.